Derivatives and Hedging Activities - Notional Amounts and Estimated Fair Values of Derivatives (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Sep. 22, 2015
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 404,900,000	$ 253,800,000
Derivative Assets, Estimated Fair Value	8,800,000	2,700,000
Derivative Liability, Notional Amount	433,000,000	153,600,000	$ 100,000,000
Derivative Liabilities, Estimated Fair Value	7,900,000	1,300,000
Not Designated as Hedging Instrument | Interest rate swap contracts
Derivative [Line Items]
Derivative Asset, Notional Amount	344,200,000	53,600,000
Derivative Assets, Estimated Fair Value	7,500,000	1,300,000
Derivative Liability, Notional Amount	344,200,000	53,600,000
Derivative Liabilities, Estimated Fair Value	7,800,000	1,300,000
Not Designated as Hedging Instrument | Interest rate lock commitments
Derivative [Line Items]
Derivative Asset, Notional Amount	60,700,000	73,400,000
Derivative Assets, Estimated Fair Value	1,300,000	1,100,000
Not Designated as Hedging Instrument | Forward loan sales contracts
Derivative [Line Items]
Derivative Asset, Notional Amount	0	126,800,000
Derivative Assets, Estimated Fair Value	0	300,000
Derivative Liability, Notional Amount	88,800,000	0
Derivative Liabilities, Estimated Fair Value	100,000	0
Designated as Hedging Instrument | Interest rate swap contracts
Derivative [Line Items]
Derivative Liability, Notional Amount	0	100,000,000
Derivative Liabilities, Estimated Fair Value	$ 0	$ 0